Hydraulic brake systems have a master brake cylinder which, in response to the actuation of a brake pedal, converts brake pedal pressure into hydraulic pressure, which is provided by means of brake fluid.
In a first known brake system, the hydraulic pressure in the master brake cylinder is provided directly to two service brake circuits. The pressure is transmitted by an intermediate service brake modulator to the service brake of the vehicle equipped with the brake system. The service brake modulator has several solenoid valves and return pumps for an antilock function. Furthermore, the service brake modulator can have additional solenoid valves which, in conjunction with the return pumps, allow an active braking pressure build-up. In general, at least four solenoid valves and two pressure vessels are required for this, together with a corresponding design of the return pumps in the service brake modulator, which is complex and expensive.
It is known that vehicles carry at least one hydraulic circuit, which has hydraulic fluid with hydraulic circuit pressure. A power steering system, for example, can be operated by means of the hydraulic fluid with the hydraulic circuit pressure.
In a second known brake system, the service brake circuit is not supplied with brake fluid by the master brake cylinder. The brake pressure provided by the master brake cylinder serves merely to actuate relay valves, by means of which the provision of brake pressure into the service brake circuits is controlled. The pressure in the service brake circuits is built up in a technically complex and costly manner by means of hydraulic pumps. An active brake pressure build-up in the service brake circuits is also possible by means of solenoid valves.